Maybe I'm a bit of a luddite, but I haven't struggled too much with just the 1.4 syntax.
Allowing objects to implement the Serializable interface (as opposed to the GWT specific IsSerializable interface) was a big plus for me, so would have been on my list if you had asked pre GWT 1.4 RC1.
The only things I am missing just now are for it to "sexed" up a little, with some sliders (we show how to write one in the book) and effects, and I guess the ability to natively add more listener objects to the document.
I don't have a wish list as I'm a new comer to GWT (I don't know the in and outs). But I would like to see extra components from Google like, hmmm let me think... Calendar, RSS reader, autocompleter, editable tables ... And Scriptaculous like effects are great too